FIG. 16 is a diagram illustrating the configuration of a conventional motion detecting circuit used in a digital TV receiver, etc. Input to input terminal 100 is video signal VS.sub.i, which is an 8-bit TV signal prepared by A/D conversion after demodulation. This input video signal VS.sub.i is sent both to one input terminal of subtractor 102 and frame memory 104 and edge detecting circuit 106. Frame memory 104 sends the video signal of the previous picture, that is, the video signal of one previous frame in the NTSC format, as delayed video signal VS.sub.d to the other input terminal of subtractor 102.
Subtractor 102 subtracts delayed video signal VS.sub.d from input video signal VS.sub.i to derive the difference in units of pixels. Difference signal e output from subtractor 102 is converted to absolute value .vertline.e.vertline. by absolute value circuit 108, and the absolute value is then sent to nonlinear processing circuit 110. Nonlinear processing circuit 110 has the nonlinear input/output characteristics shown in FIG. 17. It outputs output signal f as, for example, a 6-bit signal, corresponding to input difference absolute value .vertline.e.vertline. to one input terminal of divider 112.
Edge detecting circuit 106 passes input image signal VS.sub.i to two-dimensional high pass filter and outputs in, for example, a 4-bit signal form, edge detection signal eg which is expressed with even a fineness of edge corresponding to a determination on whether each pixel is within the edge portion (profile) portion. The signal eg is sent to the other input terminal of divider 112. At divider 102, one input signal f is divided by the other input signal eg, and the quotient h (f/eg) is output in, for example, 4-bit signal form. Output signal h of divider 112 is quantized to the desired bit number by quantization circuit 114, and the signal is output from output terminal 116 as a multivalue (for example, 2-bit) or binary (1-bit) motion signal ms.
In the static picture region, as the difference e between input video signal VS.sub.i and delayed video signal VS.sub.d of the previous picture, and its absolute value .vertline.e.vertline. is null or near null, that is, the value is smaller than threshold TH of nonlinear processing circuit 110, no significant output signal f is generated by nonlinear processing circuit 110, and the value of motion signal ms is null.
On the other hand, in the motion picture region, the absolute value of the difference .vertline.e.vertline. becomes larger than the threshold TH, and a significant output signal f is generated by nonlinear processing circuit 110, and signal ms also becomes a significant value corresponding to the magnitude of the motion. In addition, for the various portions within the motion picture region making the same motion, the difference absolute value .vertline.e.vertline. at the edge portion is larger than that at the flat portion. Consequently, division processing is performed by using edge detecting signal eg, and the sensitivity is adjusted for the motion at the various portions in the motion picture region.
However, as input video signal VS.sub.i contains the noise in the transmission system and receiving/demodulating system, difference e obtained at the output terminal of subtractor 102 also contains the noise component. In the conventional motion detecting circuit, when the noise level is high in a static picture or in the static picture region of a picture, difference absolute value .vertline.e.vertline. may exceed threshold TH of the nonlinear processing circuit 110, and a significant motion signal ms may be generated erroneously. When threshold TH is increased to eliminate this problem, when the motion in the motion picture region is small, that is, when difference e is small, difference absolute value .vertline.e.vertline. cannot exceed threshold TH, and no significant motion signal ms can be generated. This is also undesirable.
As explained above, for the conventional motion detecting circuit, depending on whether the difference absolute value of the pixel unit between the current picture (input video signal) and the preceding picture (delayed video signal) exceeds the threshold TH of nonlinear processing circuit 110, a determination is made on whether the pixel is in the motion picture region or the static picture region containing noises. For this method, small motion may be taken erroneously as noise, while large noise may be picked up as motion. This is a disadvantage.
It is an object of this invention to provide a motion detecting circuit of high reliability having the capability of correctly detecting the motion picture or motion picture region independent of the magnitude of the motion, presence/absence of noise, and magnitude of the noise.
Another object of this invention is to provide a noise reducing circuit of high reliability having the capability of performing a noise reducing procedure for the motion picture or motion picture region independent of the magnitude of motion, with noise reduction being performed only for the static picture or static picture region.